**7. The effect of the halogen bond acceptor**

When fine-tuning halogen bonding interactions it is, at least in principle, possible to modify the properties of both the halogen bond acceptor and the halogen bond donor. According to the σ-hole theory, the XB acceptor should be rich in negative electrostatic potential, or the acceptor is at least expected to be charge concentrated, which is required by lump-hole theory. Even according to the concept of amphoteric halogen bonds, the acceptor should be able to act as an electron donor. Probably the most commonly used halogen bonding acceptors are covalently bonded halogens and nitrogen atoms.[8,11,20–24] Metal halides, oxygen, sulfur, selenium and even silicon have, however, been reported to have the capacity to act as halogen bond acceptors with suitable donors.[25–31]
